The present invention relates generally to an apparatus for measuring the winding temperature of electric machines, and more particularly to apparatus for measuring the winding temperature by using an a.c. reference voltage source, a current detector and a measuring and evaluating device.
An apparatus which uses one or more sensors in the machine to measure temperature is disclosed in G. Muller, "Elektrische Maschinen" [Electric Machines], Verlag Technik, Berlin 1974. Furthermore, apparatus which have a reference voltage source and a current generated by the reference voltage, whereby a direct measurement of resistance is performed using d.c. current are disclosed in IEC Publication 279, "Measurement of the Winding Resistance of an a.c. Machine During Operation at Alternating Voltage," Geneva, 1969. The former disclosed apparatus require temperature sensors and the requisite supply lines, i.e., increased equipment. The latter disclosed apparatus can deliver erroneous measurements. D.C. voltage potentials on the mains, caused for example, by concurrently operating frequency converters or by thermoelectromotive forces caused by soldered junctions, welded points or clamping points in the mains, which have different temperatures, can greatly falsify the measurement. The possibility of blocking d.c. voltage potentials from the mains by means of capacitors is only useful in small machines. In large machines, the cost and space requirements for the capacitors are disproportionately great.
A method is also disclosed in E & I, Anno 105, Issue 7/8, pp 315 to 318, which makes possible a measurement of the rotor temperature of squirrel-cage induction machines without using thermal sensors. In this case, the rotor temperature is calculated from the variation of the voltage across terminals after switching off a machine that operating without a load and from the time constants of the rotor.
The present invention is directed to the problem of further developing an apparatus for measuring the winding temperature across the line feed of an operating electric machine while dispensing with the additional sensors, and the capacitors which are connected in series to the machine, through which the entire power received by the machine would have to flow.